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Abstract

Dye doped photocurable cholesteric liquid crystal was used to produce solid Bragg onion omnidirectional lasers. The lasers were produced by dispersing and polymerizing chiral nematic LC with parallel surface anchoring of LC molecules at the interface, extracted and transferred into another medium. Lasing characteristics were studied in carrier medium with different refractive index. The lasing in spherical cholesteric liquid crystal was attributed to two mechanisms, photonic bandedge lasing and lasing of whispering-gallery modes. The latter can be suppressed by using a higher index carrier fluid to prevent total internal reflection on the interface of the spheres. Pulse-to-pulse stability and threshold characteristics were also studied and compared to non-polymerized lasers. The polymerization process greatly increases the lasing stability.

Figures (5)

Fig. 1 (a) Dispersion of polymerized CLC droplets in glycerol. Their red color is due to DCM fluorescent dye. Larger droplets do not have a perfect spherulite structure and may contain several defects. (b) A single CLC droplet (15 µm) with a defect at the center and (c) the same droplet under crossed polarizers. (d) SEM image of a cluster of polymerized dried spheres.

Fig. 2 The lasing spectrum from a 20 µm diameter laser in glycerol. The broader equally spaced spectral peaks correspond to WGM lasing and the sharper highest peak corresponds to Bragg lasing. The upper left inset shows the reflectance measurement of a thin layer of undoped polymerized CLC mixture. The upper right inset shows unpolarized micrograph of the floating droplet. The bottom right inset shows lasing droplet without background illumination. The bright spot in the center corresponds to 3D Bragg lasing and the bright rim corresponds to the WGM lasing.

Fig. 3 Spatially resolved spectrum of the same droplet as in Fig. 2. The slit was positioned across the center of the droplet as depicted in the inset. The x-axis represents the wavelength and the y-axis represents the position along the slit. WGM lasing is seen as vertical lines across the whole diameter of the particle with maximum intensity at the edge. WGMs are easily distinguishable from the Bragg lasing that is identified as a single peak that is localized in spatial position and in wavelength (marked by a red circle).

Fig. 4 The lasing spectrum from a 16 µm diameter CLC microdroplet (inset) in high refractive index fluid (n = 1.52). The lasing from WMGs is suppressed because of lower index difference between the interior and the exterior of the particle and only a single spectral line is observed, which corresponds to 3D Bragg lasing.

Fig. 5 (a) Output energy distribution from a single 16 µm diameter CLC polymerized laser as the function of the input pulse energy. Each point represents a single pulse. The threshold is clearly visible at approximately 0.6 mJ/cm2. (b) The lasing characteristics for a 31 µm microlaser made from the non-polymerizable CLC mixture shows much more scattered points. The threshold is also not so sharply defined as for the polymerized laser.